1. Field of the Invention
The present invention relates to a backlight unit for a liquid crystal display (LCD) device, and more particularly, to a lamp guide, a backlight unit including the lamp guide and a liquid crystal display device including the backlight unit.
2. Discussion of the Related Art
Although cathode-ray tubes (CRTs) have been widely used as a display device for a television, a measuring instrument or an information terminal, it is hard to apply the CRTs to reduced electronic products in size and weight because of their heavy weight and large volume. Recently, flat panel display (FPD) such as liquid crystal display (LCD) devices and plasma display panels (PDPs) have been the subject of research and development because of their superior thin profile and light weight as compared with the CRTs.
LCD devices include a liquid crystal panel having opposite two substrates and a liquid crystal layer between the two substrates. After first and second substrates, which include an array element and a color filter layer, respectively, are formed through repetition of a thin film deposition step, a photolithographic step and an etching step, a seal pattern is formed on one of the first and second substrates. The first and second substrates may be referred to as an array substrate and a color filter substrate, respectively. Next, the first and second substrates are attached, and the liquid crystal layer is formed between the first and second substrates to complete the liquid crystal panel. After polarizing plates and a driving circuit unit are attached to the liquid crystal panel, the liquid crystal panel constitutes the LCD device with a backlight unit.
Since the LCD device is a non-emissive display device that displays images using an external light, the LCD device includes a backlight unit under the liquid crystal panel as an additional light source. The backlight unit is classified into an edge type and a direct type according to a position relation between the liquid crystal panel and a lamp of the backlight unit. Since the direct type backlight unit has no limit according to a display size of the liquid crystal panel, the direct type backlight unit has been widely used for an LCD device having a size over about 30 inches. The direct type backlight unit does not require a light guide plate (LGP) reflecting and refracting the light. In addition, the direct type backlight unit includes a plurality of lamps, a reflecting plate and a plurality of optic means. The plurality of lamps parallel spaced apart from each other are disposed under the liquid crystal panel. The reflecting plate reflects the light from the plurality of lamps toward the liquid crystal panel and the plurality of optic means are disposed over the plurality of lamps.
In the direct type backlight unit, as the liquid crystal panel is enlarged, each lamp is also elongated. Accordingly, a lamp guide fixing and supporting each lamp is required. FIG. 1 is a cross-sectional view showing a liquid crystal display device including a direct type backlight unit according to the related art. In FIG. 1, a liquid crystal panel 45 and a backlight unit 50 are integrated in an LCD device 1 by mechanical elements. Accordingly, the LCD device 1 includes the liquid crystal panel 45, the backlight unit 50, a bottom frame 15, a main frame 60 and a top frame 70. The liquid crystal panel 45 is disposed over the backlight unit 50, and the main frame 60 surrounds side surfaces of the liquid crystal panel 45 and the backlight unit 50. The top frame 70 surrounds a front edge surface of the liquid crystal panel 45, and the bottom frame 15 wraps a rear surface of the backlight unit 50. The top frame 70 and the bottom frame 15 are combined through the main frame 60.
The liquid crystal panel 45 includes a first substrate 45a, a second substrate 45b and a liquid crystal layer (not shown) between the first and second substrates 45a and 45b. Although not shown in FIG. 1, a driving circuit is connected to a side of the liquid crystal panel 45 and is bent toward a rear of the liquid crystal panel 45.
The backlight unit 50 includes a reflecting sheet 20, a plurality of lamps 25 and a plurality of optic sheets 35. The reflecting sheet 20 has a white color or a silver color and is disposed over the bottom frame 15. For example, the reflecting sheet 20 may include aluminum (Al). The plurality of lamps 25 are parallel arranged over the reflecting sheet 20, and the plurality of optic sheets 35 cover the plurality of lamps 25. For example, the plurality of lamps 25 may include a cold cathode fluorescent lamp (CCFL). In addition, the plurality of optic sheets 35 may include a prism sheet, a diffusing sheet and a protecting sheet. As a result, lights emitted from the plurality of lamps 25 and reflected on the reflecting sheet 20 are supplied to the liquid crystal panel 45 through the plurality of optic sheets 35. The brightness of the lights becomes uniform while passing through the plurality of optic sheets 35.
Although not shown in FIG. 1, the plurality of lamps 25 may be fixed and supported by a side supporter at both ends thereof. In a direct type backlight unit, as the liquid crystal panel 45 is enlarged, each lamp 25 is elongated. Accordingly, a lamp guide fixing and holding each lamp 25 is formed at intermediate portions between the both ends thereof to prevent a droop or a warpage of each lamp 25 at the intermediate portions.
FIG. 2 is a cross-sectional view showing a direct type backlight unit of a liquid crystal display device according to the related art. In FIG. 2, a reflecting sheet 20 and a lamp guide 40 are sequentially disposed over a bottom frame 15. The bottom frame 15 includes a plurality of first through holes TH1 and the reflecting sheet 20 includes a plurality of second through holes TH2 corresponding to the plurality of first through holes TH1. The lamp guide 40 is coupled with the bottom frame 15 through the plurality of first through holes TH1 and the plurality of second through holes TH2. A plurality of lamps 25 are hold and fixed by the lamp guide 40 and a plurality of optic sheets 35 are disposed over the plurality of lamps 25.
The lamp guide 40 includes a horizontal part 42, a supporting part 44, a plurality of holding part 46 and first and second coupling parts 47 and 48. Since the supporting part 44 having a cone shape upwardly extends from a front surface of the horizontal part 42 to support the plurality of optic sheets 35, the distance between each lamp 25 and the plurality of optic sheets 35 is kept uniform. The plurality of holding parts 46 are formed on the front surface of the horizontal part 42, and each holding part 46 holds and fixes each lamp 25. In addition, each holding part 46 has a ring shape having an open portion and may include an elastic material such as a plastic to hold each lamp 25 strongly.
Each of the first and second coupling parts 47 and 48 downwardly extends from a rear surface of the horizontal part 42. For example, each of the first and second coupling parts 47 and 48 may have a trident shape having a hanging portion and may include an elastic material such as a plastic. Accordingly, after each of the first and second coupling parts 47 and 48 is inserted through the first and second through holes TH1 and TH2, the lamp guide 40 is strongly coupled with the reflecting sheet 20 and the bottom frame 15 due to the elastic hanging portion.
The procedure of coupling the bottom frame 15, the reflecting sheet 20, the lamp guide 40 and the plurality of lamps 25 will be illustrated hereinafter. After the bottom frame 15 is aligned to the reflecting sheet 20 such that the plurality of first through holes TH1 correspond to the plurality of second through holes TH2, each of the first and second coupling means 47 and 48 of the lamp guide 40 is inserted into the first and second through holes TH1 and TH2 to couple the lamp guide 40 with the reflecting sheet 20 and the bottom frame 15. In addition, each of the plurality of lamps 25 is inserted into each of the plurality of holding parts 46 of the lamp guide 40 coupled with the reflecting sheet 20 and the bottom frame 15.
The horizontal part 42 of the lamp guide 40 is spaced apart from the reflecting sheet 20 by a gap G to prevent deterioration of the reflecting sheet 20 such as a scratch due to the lamp guide 40. For example, the gap may be within a range of about 0.5 mm to about 1 mm. Accordingly, when the plurality of lamps 25 are inserted into the plurality of holding parts 46, the horizontal part 42 of the lamp guide 40 may be pressed down and may be warped in the gap G to contact the reflecting sheet 20. The deformation such as a warpage of the horizontal part 46 may cause damage such as a crack to each lamp 25, and the contact between the horizontal part 42 and the reflecting sheet 20 may cause deterioration such as a scratch of the reflecting sheet 20. As a result, production efficiency is reduced to prevent the warpage or the contact.
In addition, the reflecting sheet 20 may be formed of a micro cellular polyethylene terephthalate (MCPET) having a thickness of about 0.6 mm to about 1 mm. Although the reflecting sheet 20 of MCPET has a relatively high reflectance, the reflecting sheet 20 of MCPET has a disadvantage of a relatively high cost. Accordingly, a reflecting sheet 20 having a reduced thickness of about 0.2 mm to about 0.3 mm has been researched and developed. Although the reflecting sheet 20 of the reduced thickness has no problem under a temperature of about 10° C. to about 30° C., the reflecting sheet 20 of the reduced thickness may be deteriorated under a relatively low temperature or under a relatively high temperature. For example, the reflecting sheet 20 of the reduced thickness may have a warpage or a wrinkle. However, since the horizontal part 42 of the lamp guide 40 is spaced apart from the reflecting sheet 20 of the reduced thickness, the lamp guide 40 does not press down the deteriorated portion of the reflecting sheet 20 under the relatively low temperature or under the relatively high temperature. As a result, the effective reflectance of the reflecting sheet 20 is reduced due to scattering at the deteriorated portion or the lamp guide 40 contacts the deteriorated portion of the reflecting sheet 20 to generate a defect in the reflecting sheet 20.